The production of a stable, white lubricating oil of high viscosity index in good yield from a heavy lubricating oil distillate normally requires a two-stage process utilizing two different catalysts. The first stage is employed to effect the primary charge stock alterations, i.e., saturation of aromatics, cracking naphthenes, and isomerization of paraffins. A significant portion of the nitrogen and sulfur are also removed in the first stage. The second stage, which follows an interstage dewaxing of the first stage effluent, is a mild hydrogenation step employing a catalyst which is sensitive to high concentrations of hydrogen sulfide and ammonia. In this stage, additional amounts of sulfur are removed and light stability is imparted by further reduction in the levels of nitrogen and aromatics.
To produce an acceptable first-stage product, the catalyst must have high hydrogenation and isomerization activities but without the extensive hydrocracking that would result in excessive loss of material from the lubricating oil boiling range. It should also have good activity for nitrogen and sulfur removal and at the same time remain active in the presence of high partial pressures of hydrogen sulfide and ammonia.
Various catalyst systems have been used for upgrading lubricating oils including hydrotreating catalysts of the type described in U.S. Pat. Nos. 3,078,221 and 3,078,238 to Beuther et al, which involve the provision of a Group VI metal and Group VIII metal, such as a fluorine containing sulfided nickel and tungsten on a silica-aluminia carrier. Such catalysts have found wide commercial acceptance. More recently, an improved hydrotreating catalyst having a magnesia-alumina-aluminum phosphate support is described in U.S. Pat. No. 4,382,877 to Kehl. The Kehl catalyst possesses improved hydrodenitrification activity which is important for increasing catalyst life in the second stage of a two-stage hydrotreating process, since the second stage catalyst is normally more nitrogen-sensitive.